Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a medium casserole dish.

Cook spinach according to package directions. Place potatoes in a pot with enough water to cover, and bring to a boil. Cook 15 minutes, or until tender but firm. Drain, cool slightly, and mash.

Mix spinach, mashed potatoes, butter, sour cream, onion, salt, and dill together in a bowl. Transfer to the prepared casserole dish. Top with Cheddar cheese.

Bake in the preheated oven until bubbly and lightly brown, about 20 minutes.